1. Field of the Invention
The present invention generally relates to a lens systems for devices such as digital cameras and, more particularly, to a lens system that has optical elements with aspheric surfaces.
2. Discussion of the Related Art
Digital cameras utilizing high-resolution electronic imaging sensors typically require high-resolution optical elements such as lenses. In addition, the lenses generally must be very compact, so that they can be incorporated into devices such as palm-sized computers, cellular telephones, and the like.
Lenses for digital cameras generally have a plurality of individual lens elements. The lens elements are typically spherical and so usually create spherical aberration. Chromatic aberration, coma aberration, distortion, and field curvature are also common optical aberrations that occur in the imaging process of a typical lens. A large number of lens elements are generally required in order to balance these inherent optical aberrations. Lenses having a large number of lens elements tend to be large, heavy, and expensive to manufacture. This greater manufacturing cost is caused by the added cost of assembling and mounting the lens elements into a lens barrel as well as the materials used in their construction.
Furthermore, conventional lenses commonly use one or more aspheric lens elements, each of which has one or two non-spherical surfaces. The aspheric lens elements are usually made of plastic or glass. Aspheric plastic lens elements may be produced by means of plastic injection molding and are therefore relatively inexpensive. However, the optical properties of most plastic lens elements are highly sensitive to changes in temperature and humidity, such as when the digital camera is used outdoors on very hot or cold days. On top of this, the hardness of optical plastic material is lower than that of an optical glass material, so the surfaces of such lens elements are easily scraped or abraded, which can also affect image precision. In comparison, glass aspheric lens elements have good optical properties and scratch-resistant. However, glass aspheric lenses cannot be easily produced by traditional glass grinding and polishing techniques. In addition, glass lens elements are heavier than plastic lens elements and thus defeats the goal of producing more light-weight digital cameras.
Thus it can be seen that a typical lens system has both spherical lens elements and aspheric lens elements. The lens system includes a first spherical lens element, a second spherical lens element, and a third lens element. The first lens element and the second lens element are made of glass. The third lens element has two aspheric surfaces and is made of plastic. Although the typical lens may satisfy the requirements for imaging, the resolution of the lens is low and may affect the image performance.
Accordingly, what is needed is a lens system for a digital camera which is compact and which provides good imaging quality.